What is this and how do I stop it?

This pipe looks like it was cut off at one time and plugged with something. I'm not sure where it goes or whether it's a radiator pipe or an old water pipe. It doesn't appear to go anywhere on the first floor of my house. (This is in the basement). The only thing remotely close to it is a radiator on the 2nd floor (assuming it goes all the way up) but that radiator is working so it wouldn't make any sense that it would be for that (at least to me).

Anyway, it's leaking and I'm looking for the best way to stop it. Is there some type of cap I can get for it? I'm assuming it's an iron pipe, but there are no threads to attach it to.

Could be a vent I guess. I pushed on the plastic piece that's shoved in and it's loose. Doesn't appear to be under pressure. The boiler has been running for a while now, so it's pressurized, and obviously the water is on. I'm thinking just a rubber cap with steel band would do. Let me know if anyone thinks otherwise.

IF you cap it, and there is water entering the pipe causing the leak, the pipe will fill up and then the water will have to go somewhere, possibly causing another leak elsewhere. Your REAL problem is to find where the water is coming from, not to eliminate the symptom of water leaking out of the pipe.